When a plant or animal is moved from its native homeland to a new territory, and it thrives to the detriment of native plants and animals in that territory, it’s known as an invasive species. Invasive plants in the Northeast – including common and European buckthorn, Japanese knotweed, and wild chervil – are all not invasive plants in their native territories. They play nicely with the other kids on the playground. In the Northeast, however, they’re like the new kid on the block who discovers his new playmates are much weaker than his old ones, and who becomes a bully overnight.
Ecologists aren’t exactly sure why invasive plants, relatively benign in their home ranges, become so abundant and competitive in adopted territories. Most point to biogeographical explanations – that the invader is either a stronger competitor or faces weaker competition in its new home than in its old.
There’s yet another explanation that scientists have floated, and it’s known as the “novel weapons” hypothesis. It says that some plants excrete chemicals that suppress their neighbors (think black walnut, which excretes the highly allelopathic chemical juglone), and in some cases, those chemicals are entirely novel – and therefore extremely effective against competitors – in new territories (think gunpowder, or the atomic bomb). This can manifest as a direct toxic interaction between plants, or as a secondary effect, where the invader’s toxic excretion indirectly affects a competitor, via a third party.
According to Ragan Callaway at the University of Montana and his colleagues, this is exactly what happens in the case of garlic mustard. Garlic mustard is an aggressive invasive species in the Northeast, especially in forest understories. It’s tough, spreads easily, and suppresses the growth of other herbaceous understory plants by releasing a natural herbicide.
Callaway and his research team knew that garlic mustard’s chemical suppresses mycorrhizal fungi, which are essential to the growth of many forest plants, including sugar maples. “Mycorrhizae” is the term for a symbiotic association between a fungus and the roots of a plant: the underground fungus taps into a tree’s roots to gain the benefits of photosynthesis (carbohydrates, glucose), which the tree tolerates since the fungus is handily breaking down and absorbing soil nutrients for it.
But when a garlic mustard moves in, it suppresses the growth of mycorrhizae, which in turn affects the maples. Callaway’s team wondered if this interaction happened in garlic mustard’s native Europe, or whether it was a new-kid-on-the-block phenomenon. They took soil from four sites in North America and two in the invader’s native Europe. Some of the soil from each site was planted with garlic mustard (essentially, the soil was “trained” with garlic mustard), while some was left unplanted. Next, native, mycorrhizal- and non-mycorrhizal-dependent plants from each geographical soil-sample region were harvested and grown in both the trained and untrained soils.
The researchers found equal numbers of fungal spores in both types of soils, but their viability and ability to infect their plant partners was severely inhibited in the North American soils compared with the European soils. This suggests that the European fungi, which evolved with garlic mustard, are much more tolerant of garlic mustard’s chemicals than are the un-indoctrinated North American fungi. They also found growth of North American mycorrhizal-dependent plants in trained soil was inhibited, when compared with growth of European plants in trained soil. Non-mycorrhizal-dependent plants were not affected.
This suggests that garlic mustard indeed possesses a novel weapon in North America that, along with the fact that it is tough and easily spread, makes it an especially potent invasive. It also suggests we may need to take a closer look at indirect effects of invasives on native plant and animal communities to assess their full impact.